1. Field of the Invention
The present invention relates generally to sealing devices for sealing piston and cylinder assemblies and the like, and more particularly to a stepped piston ring seal and ring expander combination for use within an annular groove in a piston employed in high pressure hydraulic applications. An important embodiment of the present invention is directed to a piston ring seal having stepped ends, which stepped ends present apposed, mating surface contours one to another such as to minimize wear on the underlying ring expander.
2. Background of the Invention
Piston ring seal and expander combinations are used to prevent leakage around a cylindrically shaped element coaxially mounted in a cylindric chamber. In the example of a hydraulic cylinder used in high pressure applications--for example, an excavator equipped with hydraulic cylinders to actuate movements of a boom, arm, thumb and bucket attached to the excavator--a ring seal is mounted coaxially in an annular groove on a piston head to prevent leakage of oil in the space between the piston head and the cylinder chamber in which it is mounted. The ring seal is circumferential, having an inner and outer diameter, a width between the inner and outer diameters, and an axial depth. The relatively stiff ring seal is generally of rectangular cross section. Also mounted in the piston groove is a ring-shaped, resilient, elastomeric expander, which underlies the ring seal and urges the ring seal radially outward toward the cylinder wall to minimize leakage.
A ring seal is conventionally manufactured initially with a continuous circumference. The circumference is thereafter severed, thus creating circumferentially apposed ends that may be separated to facilitate placing the ring into the piston groove. The apposed ends are commonly stepped to reduce leakage between the ends. One common way to create stepped circumferential ends in a continuous circumference ring seal for high pressure hydraulic applications is as follows: a first radial cut is made from the outer to the inner diameter that extends axially from one side of the ring seal half way toward the opposite side; a second radial cut is made from the outer to the inner diameter, circumferentially spaced apart from the first radial cut, that extends axially half way from the opposite side toward the first side; and a cut is made in an axial plane mid-way between the sides of the seal ring that extends circumferentially from the first radial cut to the second radial cut. Installed within a piston groove, the stepped ends overlap one another in order to reduce leakage between the ends. The circumferential extent of the overlapped, stepped ends is sufficient that an overlap is maintained even as increasing pressure within the cylinder forces the stepped ends circumferentially apart.
A continuing problem with stepped-end piston ring seal and expander combinations has been the high failure rate of the expanders. Repetitive expansion and contraction of a ring seal associated with widely varying hydraulic pressures within a hydraulic cylinder causes an extrusion of the expander into a gap between the stepped ends with a consequent nibbling and wearing away of the underlying elastomeric expander by the stepped ends.
A variety of shapes have been described for overlapping ends for piston seal rings. Examples include: Tsuge, U.S. Pat. No. 4,449,721, which disclosed a split piston ring having mating, complementary, stepped arcuate ends; Kurkowski, U.S. Pat. No. 5,087,057, which disclosed a seal ring that includes a helical cut that extends radially and circumferentially to provide cut surfaces that overlap in intimate contact; and Grimm, which disclosed a split seal ring having a pair of circumferentially projecting elements that defined offset, oppositely facing sealing surfaces, which surfaces were inclined and converged toward the outer circumferential face of the ring. These disclosures, however, fail to address the problem of nibble and excessive wear on an elastomeric expander.
Thrane et al., U.S. Pat. No. 4,206,930, however, did address the problem of expander nibble and wear by use of a stepped joint piston ring manufactured with an oversized closed gap condition such that installation of the ring into a piston groove within a cylinder compressed the ring sufficiently to close the gap between the stepped ends of the ring. The ring had a modulus of elasticity preferably between 1,000,000 and 3,000,000 pounds per square inch, and heat deflection temperature between 300.degree. and 600.degree. F. The ring was used in combination with an underlying, resilient expander within the piston groove, which expander was preferably constructed of a nylon (polyamide) material that was glass fiber filled and had a modulus of elasticity of about 1,300,000 pounds per square inch, and had a heat deflection temperature of about 485.degree. F. at 264 psi.
In view of the still existing need for improved seals, particularly in high pressure hydraulic applications, an object of the present invention is to provide an improved seal for variable pressure applications.
Another object is to provide an improved seal for a piston, rod, or the like.
Another object is to provide an improved seal that includes an outer, relatively stiff seal ring and an inner, resilient, elastomeric expander that acts to urge the outer ring into close contact with the sealed surface.
Another object is to provide an improved seal that substantially eliminates the problem of the stepped ends of the ring seal nibbling and wearing away the expander.
Still another object is to provide a piston or rod seal in which the relatively stiff ring seal portion thereof has axial depth and width that extends from an inner to an outer diameter, and contains an end gap seal that includes a pair of overlapping, oppositely directed, stepped ends, each stepped end having adjacent circumferentially-extended and circumferentially-recessed surfaces in complementary, mating relationship to the opposed surfaces of the opposite end, each mating surface being curvilinearly deviated from a radial direction to a first circumferential direction starting at a diameter intermediate the inner and outer diameters, and each mating surface along the circumferentially-recessed surface thereof being likewise deviated from a radial direction from an intermediate diameter but to a second, opposite circumferential direction.